Experimental Study On Mechanical Properties And Microstructure Of Limestone Manufactured Sand Concrete After High Temperatures

At present,the fire of highway bridges and buildings has become one of the most common disasters.As a new type of environmental protection building material,manufactured sand(MS)has broad application prospects from the perspective of ecological environment,economic benefits and green development strategy.With the wide application of manufactured sand concrete(MSC)in practical engineering,the probability of MSC structure suffering from fire is further increased.In order to research the performance of MSC after high temperature,this paper takes the stone powder content and temperature as the research variables,and the main research contents are as follows:Through the tests of compressive strength and splitting tensile strength of MSC after high temperature,the effect mechanism of temperature and stone powder content on the strength of MSC after high temperature was analyzed.The degradation models of compressive strength and splitting tensile strength after being subjected to high temperature of MSC were established.At the same time,based on the maximum fire temperature and mass loss rate,the evaluation formulas of compressive strength and splitting tensile strength of MSC after high temperature were proposed.The results indicated that with the increase of temperature,the surface color of MSC specimens change from gray to reddish brown,and finally to gray.The mass loss rate of the specimen increases significantly with the increase of stone powder content.The compressive strength and splitting tensile strength of concrete decreased significantly with the increase of temperature.The compressive strength and splitting tensile strength of the MSC specimens firstly increased,then decreased when the stone powder content increased.And the concrete strength goes for a maximum at10%.The deterioration model and the evaluation formulas of compressive strength and splitting tensile strength of MSC after high temperature based on the measured results is relatively good with a high goodness-of-fit.Through the uniaxial compression test of MSC after high temperature,the effects of stone powder content and temperature on the stress-strain curve,peak stress,peak strain,elastic modulus,damage evolution process and energy consumption of the specimens were studied and analyzed.The results showed that the stress-strain curves tended to be flatter with the temperature increased.In addition,the peak stress,peak strain and elastic modulus of the MSC specimens were increased firstly and then declined with the increase of stone powder content.When the content of stone powder was 10%,the peak stress,peak strain and elastic modulus of MSC reached the maximum value.The complete stress-strain curve of MSC at different temperatures based on the model of Sargin was better.The initial damage value of the specimen increases with the increase of temperature.After high temperature,MSC can absorb more energy through its own deformation,especially when the stone powder content was 10%,the energy consumption coefficient was more easily affected by high temperature.The phase composition,morphology and pore structure of MSC samples before and after high temperature were investigated by X-ray diffraction,scanning electron microscope and mercury intrusion test.The results showed that the addition of stone powder could increase the amount of ettringite crystal(AFt)and calcium hydroxide(CH)in the system.Moreover,the total pore volume,porosity,average pore size,critical pore size and most probable pore size of the specimens decreased significantly.When the temperature reached500 ℃,CH was decomposed into a large number of small crystals by heat.The hydrated calcium silicate gel(C-S-H)caused smaller voids and the pore structure parameters increased significantly due to dehydration.When the temperature reached 700 ℃,CH completely dehydrated and decomposed,the surface edges and corners of needle-like AFt disappeared.In addition,the number of holes and through cracks in concrete increased sharply,and the harmful and multi harmful holes account for about 84% of the total pores,which aggravated the overall damage of concrete structure.